Proteins that selectively bind to selected targets by way of non-covalent interaction play a crucial role as reagents in biotechnology, medicine, bioanalytics as well as in the biological and life sciences in general. Antibodies, i.e. immunoglobulins, are a prominent example of this class of proteins. Despite the manifold needs for such proteins in conjunction with recognition, binding and/or separation of ligands/targets, almost exclusively immunoglobulins are currently used.
Additional proteinaceous binding molecules that have antibody-like functions are the members of the lipocalin family, which have naturally evolved to bind ligands. Lipocalins occur in many organisms, including vertebrates, insects, plants and bacteria. The members of the lipocalin protein family (Pervaiz, S., & Brew, K. (1987) FASEB J. 1, 209-214) are typically small, secreted proteins and have a single polypeptide chain. They are characterized by a range of different molecular-recognition properties: their ability to bind various, principally hydrophobic molecules (such as retinoids, fatty acids, cholesterols, prostaglandins, biliverdins, pheromones, tastants, and odorants), their binding to specific cell-surface receptors and their formation of macromolecular complexes. Although they have, in the past, been classified primarily as transport proteins, it is now clear that the lipocalins fulfill a variety of physiological functions. These include roles in retinol transport, olfaction, pheromone signaling, and the synthesis of prostaglandins. The lipocalins have also been implicated in the regulation of the immune response and the mediation of cell homoeostasis (reviewed, for example, in Flower, D. R. (1996) Biochem. J. 318, 1-14 and Flower, D. R. et al. (2000) Biochim. Biophys. Acta 1482, 9-24).
Lipocalins share unusually low levels of overall sequence conservation, often with sequence identities of less than 20%. In strong contrast, their overall folding pattern is highly conserved. The central part of the lipocalin structure consists of a single eight-stranded anti-parallel β-sheet closed back on itself to form a continuously hydrogen-bonded β-barrel. This β-barrel forms a central cavity. One end of the barrel is sterically blocked by the N-terminal peptide segment that runs across its bottom as well as three peptide loops connecting the β-strands. The other end of the β-barrel is open to the solvent and encompasses a target-binding site, which is formed by four flexible peptide loops. It is this diversity of the loops in the otherwise rigid lipocalin scaffold that gives rise to a variety of different binding modes each capable of accommodating targets of different size, shape, and chemical character (reviewed, e.g., in Flower, D. R. (1996), supra; Flower, D. R. et al. (2000), supra, or Skerra, A. (2000) Biochim. Biophys. Acta 1482, 337-350).
Various PCT publications (e.g., WO 99/16873, WO 00/75308, WO 03/029463, WO 03/029471 and WO 2005/19256) disclose how muteins of various lipocalins (e.g. NGAL lipocalin) can be constructed to exhibit a high affinity and specificity against a target that is different than a natural ligand of a wild type lipocalin. This can be done, for example, by mutating one or more amino acid positions of at least one of the four peptide loops. In addition, PCT publication WO 2012/022742 teaches methods for generation of lipocalin muteins directed against hepcidin.
Hepcidin, a peptide hormone typically existing in two forms made of either 20 or 25 amino acids, produced predominantly in hepatocytes of the liver, plays a central role in the regulation of iron homeostasis, acts as an antimicrobial peptide and is directly or indirectly involved in the development of most iron-deficiency/overload syndromes. A major action of hepcidin is to internalize and degrade the iron exporter ferroportin, which is expressed on all iron-exporting cells. Hepcidin directly binds to ferroportin. A low concentration of hepcidin level leads to acceleration of iron release from macrophages and hepatocytes.
Methods of isolating, analyzing and quantifying hepcidin as well as agents for the treatment of diseases and/or conditions associated with decreased levels of iron have been described in international patent applications WO 2008/011158, WO 2008/097461, WO 2009/094551A1, WO 2009/139822, WO 2009/058797 and WO 2010/017070. However, no protein having the features attendant to the proteins provided by present disclosure has been previously described.
Therefore, it would be desirable to have improved therapeutic methods involving therapeutically effective amount of a composition comprising at least one mutein of human NGAL lipocalin, which is capable of increasing the bioavailability of iron in a body fluid such as blood and exhibits in vivo therapeutic activities in a subject in need thereof.